No rest for the laurels: symbiotic invaders cause unprecedented damage to southern USA forests

Laurel wilt is an extraordinarily destructive exotic tree disease in the southeastern United States that involves new-encounter hosts in the Lauraceae, an introduced vector (Xyleborus glabratus) and pathogen symbiont (Raffaelea lauricola). USDA Forest Service Forest Inventory and Analysis data were used to estimate that over 300 million trees of redbay (Persea borbonia sensu lato) have succumbed to the disease since the early 2000s (ca 1/3 of the pre-invasion population). In addition, numerous native shrub and tree species in the family are susceptible and threatened in the Western Hemisphere. Genetic markers were used to test the hypothesis that the vector and pathogen entered North America as a single introduction. With a portion of the cytochrome oxidase I gene, a single X. glabratus haplotype was detected in the USA. Similarly, Amplified Fragment Length Polymorphisms indicated that 95% (54 of 57) of the isolates of R. lauricola that were examined were of a single clonal genotype; only minor variation was detected in three polymorphic isolates. Similar levels of disease developed after swamp bay (P. palustris) was inoculated with each of the four genotypes of R. lauricola. It is proposed that a single founding event is responsible for the laurel wilt epidemic in the United States.